Carbon dioxide is a chemical chimera. Essential to life by virtue of its role as the carbon supplier in photosynthesis, it has also come to be regarded as an environmental threat due to its contributions to global warming. Likewise, while certain recent work has constructively harnessed CO2 to form novel soft materials, other current research focuses on the synthesis of new materials explicitly for achieving CO2 removal when its presence constitutes a nuisance.
The reversible capture of CO2 is a process of importance in applications ranging from respiration devices to natural gas sweetening (Stewart, C.; Hessami, M.-A. Energy Conv. Mgmt., 2005, 46, 403; Harrison, D. P.; Silaban, A. Chem. Eng. Comm. 1995, 137, 177). For example, the reversible capture of CO2 is a prominent feature of schemes for the mitigation of causative agents in global warming. Already important in the purification of natural gas and in breathing-air recirculation systems, CO2 capture is achieved on large scales by passing a contaminated gas through an aqueous amine solution, with which the entrained CO2 reacts (Kohl, A.; Nielsen, R. Gas Purification, 5th ed., Gulf: Houston, 1997; Chapters 1, 2. and 14). Unfortunately, the process is frustrated by the volatility of the dissolved amines, which are gradually lost into the gas stream. Accordingly, if reactive capture is to be an element of future CO2 management technologies, there is a pressing need to develop systems in which the scavenger is both affordable and non-volatile.
One of the most promising new categories of materials for use in CO2 removal is ionic liquids (ILs). At higher pressures, CO2 has a greater innate solubility in many classical ILs than do other gases, making physical solvation a potential method of removal. At lower pressures, reactive gas capture by amine-functionalized task-specific ionic liquids (TSILs) is promising. Using these functional salts, it is possible to capture CO2 in a fashion akin to commercial scrubbing amines while avoiding backpressure from the amine and the slow loss of the amine into the treated gas stream (Bates, E. D. et al. J. Am. Chem. Soc. 2002, 124, 4194). Since ionic liquids (ILs) typically lack a detectable vapor pressure, they are conceptually ideal materials for CO2 capture technology (Bates, E. D. et al. J. Am. Chem. Soc. 2002, 124, 4194; Zhang, J. et al. Chem. Eur. J. 2006, 12, 4021; Huang, J. et al. Chem. Comm. 2006, 4027; Anthony, J. L. et al. Int. J. Envir. Tech. Mgmt. 2004, 4, 105). However, the scales involved in industrial capture applications require large amounts of the reactive agents. Moreover, some amine-functionalized TSILs are relatively costly and/or tedious to prepare and purify. Further, these TSILs have or potentially have problems with long-term stability.
Accordingly, it is important to find new CO2-reactive TSILs and related soft ionic materials, especially by co-opting commercially available commodity chemicals as starting materials, and assembling them in quick, high-yielding and atom-efficient reactions. Furthermore, since the ultimate goal is the development of property-tunable ionic materials for large-scale CO2 scavenging, it is vital to identify alternatives which can be made using a procedure that is uncomplicated, economically attractive, and capable of quickly producing a large number of materials for screening purposes. Specifically, an approach which embodies attributes of the Kolb, Finn and Sharpless “click” concept (rapid, modular, employing commodity chemicals, highly atom efficient, using minimal and/or relatively benign solvents, and giving high yields of products useable with little or no purification) is desired (Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem. Int. Ed. 2001, 40, 2004).